Toroidal continuously variable transmissions that are used as automatic transmissions for automobiles are disclosed in many publications such as JP 2001-317601 (A) and “Toroidal CVT”, Hirohisa Tanaka, Corona Publishing Co., Ltd. (Jul. 13, 2000), and such transmissions are well known and are partially being put into use. This kind of toroidal continuously variable transmission includes an input-side disk and an output-side disk that are such that the side surfaces in the axial direction thereof that face each other are toroid shaped curved surfaces, and a plurality of power rollers that is held between these disks. During operation, the rotation of the input-side disk is transmitted to the output-side disk by way of the power rollers. These power rollers are supported by support members such as trunnions so as to be able to rotate freely, and these support members are supported so as to be able to freely pivot and displace around pivot shafts that are located offset from the center axis of the input-side and output-side disks such that the pivot shafts and the center axis do not lie in the same plane. When changing the transmission gear ratio between the input-side and output-side disks, a hydraulic actuator causes the support members to displace in the axial direction of the pivot shafts. Supply and discharge of hydraulic oil to and from this actuator is controlled by a control valve, however, at the same time, the movement of the support members is fed back to the control valve.
When the support members are caused to displace in the axial direction of the pivot shafts based on the supply and discharge of hydraulic oil to and from the actuator, the direction of the force in the tangential direction that acts at the areas of contact (traction sections) between the circumferential surfaces of the power rollers and the side surfaces of the input-side and output-side disks changes, and side slipping occurs in the areas of rolling contact. As the direction of this force changes, each of the support members pivots (inclines) around the respective pivot shaft, and the locations of contact between the circumferential surfaces of the power rollers and the side surfaces of the input-side and output-side disks change. When the circumferential surfaces of these power rollers come in rolling contact with the portion of the input-side disk that is near the outside in the radial direction and the portion of the output-side disk that is near the inside in the radial direction, the transmission gear ratio between the input-side and output-side disks is on the accelerating side. On the other hand, when the circumferential surfaces of these power rollers come in rolling contact with the portion of the input-side disk that is near the inside in the radial direction and the portion of the output-side disk that is near the outside in the radial direction, the transmission gear ratio between the input-side and output-side disks is on the decelerating side.
When this kind of toroidal continuously variable transmission device is assembled in an automatic transmission of an automobile, construction in which the continuously variable transmission is combined with a differential gear unit such as a planetary gear mechanism has been proposed. JP 2003-307266 (A) discloses a continuously variable transmission device in which the input shaft is rotated in one direction, and the rotating state of the output shaft is switched between a forward rotating state and a reverse rotating state with the stopped state (so-called gear-neutral state) in between. In the case of this kind of continuously variable transmission device, in the so-called low-speed mode state, the transmission gear ratio of the overall continuously variable transmission device changes to become infinitely large. In other words, by adjusting the transmission gear ratio of the toroidal continuously variable transmission, while the input shaft remains in a state of rotating in a single direction, the rotating state of the output shaft can be changed between a forward rotating state and reverse rotating state with a stopped state in between. In the case of a continuously variable transmission device that is capable of achieving an infinitely large transmission gear ratio, the transmission gear ratio of a toroidal continuously variable transmission is such that near a value where it is possible to achieve a stopped state of the output shaft (geared neutral point, GN point), the state of the power that is transmitted to the output shaft greatly changes even when this transmission gear ratio is changed only a little. Therefore, control of the transmission gear ratio of a toroidal continuously variable transmission must be performed with high precision.
For example, when the automobile is in the stopped state and the shift lever is moved from a non-moving state such as the P range (parking position) or N range (neutral position) to a moving state such as the D range (normal forward position), L range (high drive forward position) or R range (reverse position), a suitable driving force in the forward or reverse direction is quickly generated and it is necessary to keep the vehicle in the stopped state by a braking force caused by operating the brake pedal. Therefore, in a state in which the shift lever has selected a non-moving state, the transmission gear ratio of a toroidal continuously variable transmission must be strictly controlled at a value at which it is possible to achieve a state of an infinitely large transmission gear ratio. Supposing that the transmission gear ratio of a toroidal continuously variable transmission shifts a large amount from a value at which it is possible to achieve an infinitely large transmission gear ratio, and the shift lever has selected a moving state, there is a possibility that a driving force that is greater than anticipated (creep force) will be transmitted and the vehicle will begin to move, or that a driving force in a direction opposite that intended by the operator will be transmitted.
On the other hand, there is a large number of parts assembled in a toroidal continuously variable transmission, and the dimensional precision and assembly precision of many of those parts have an effect on the transmission gear ratio of the toroidal continuously variable transmission. Therefore, it is feasible that individual differences will occur in the transmission gear ratio of toroidal continuously variable transmissions that are capable of achieving a state of an infinitely large transmission gear ratio that is found through design calculation. Moreover, it is also feasible that the characteristics of a transmission gear ratio of a toroidal continuously variable transmission that is capable of a state of an infinitely large transmission gear ratio will change due to changes over time of the components that are used for long periods of time (slight plastic deformation).
On the other hand, JP 2004-308853 (A) discloses giving a learning function to a controller for learning the step position of a stepping motor, in which with the shift lever selecting a non-moving state as a condition, the output shaft is stopped with the input shaft rotating as is. More specifically, with the shift lever selecting a non-moving state as a condition, the rpm of the input-side disk and the rpm of the output-side disk of a toroidal continuously variable transmission are detected by respective rotation sensors. The controller finds the rotational speed of the output shaft in a non-moving state based on the actual transmission gear ratio that is obtained from the rotational speeds of the input-side disk and output-side disk (rotational speed of the input-side disk/rotational speed of the output side disk), and the transmission gear ratio of a planetary gear transmission. The controller then adjusts the transmission gear ratio of the toroidal continuously variable transmission by adjusting the step position (driving amount) of the stepping motor so that the rotational speed of the output shaft is “0”. The controller learns the step position for the state where the rotational speed of the output shaft is “0”, then stores that step position in the controller memory to complete learning control. The controller controls the transmission gear ratio of the toroidal continuously variable transmission with the adjusted step position (learned value) as a reference. As a result, it becomes possible to control the transmission gear ratio with high precision without being affected by individual differences in or changes over time of the components of the toroidal continuously variable transmission.
However, in the case of the control method related to learning the step position that was conventionally considered, there is a possibility that when the engine rotational speed fluctuates, and the rotating state of the input-side disk and output-side disk that are used for learning the step position becomes unstable, the step position will not be accurately learned. For example, in a state where the operator continuously steps on the accelerator pedal and releases the accelerator pedal (pumps the accelerator pedal), or a state where the amount that the accelerator pedal is pressed is changed (increased or decreased), or a state immediately after releasing the accelerator pedal, the engine rotational speed fluctuates even when the shift lever has selected a non-moving state. When the engine rotational speed fluctuates in this way, the actual transmission gear ratio of the toroidal continuously variable transmission does not change, however, there is a possibility that the calculated transmission gear ratio of the toroidal continuously variable transmission, which is calculated by the controller from the rotational speeds of the input-side disk and output-side disk will change or become unstable due to the detection timing of the sensor for detecting the rotational speeds of the input-side disk and output-side disk and the control period (detecting period of rotational speed) of the controller (engine control unit, ECU). Therefore, there is a possibility that the time (learning time) required for the transmission gear ratio of the toroidal continuously variable transmission to be adjusted to a value (range) that can achieve a state of an infinitely large transmission gear ratio will become long, or that there will be deviation in the learned valued for the step position. As a result, with the step position of the stepping motor shifted from the proper position for stopping the output shaft, there is a possibility that control of the transmission gear ratio will start, and the feeling of shifting is lost, or in the worst case, there is also a possibility that the vehicle will move in a direction opposite from the direction selected by the shift lever position, so in order to prevent the occurrence of such a condition, it is necessary to have a separate failsafe mechanism. However, even where there is such a failsafe mechanism, without performing backup control when the engine rotational speed becomes unstable, it is difficult to sufficiently ensure reliability of the learned value for the step position of the stepping motor of this kind of continuously variable transmission device.